a) Field of the Invention
The present invention relates to a position detecting apparatus and a position detecting method for position alignment. More particularly, the invention relates to a position detecting apparatus using an observation optical system having an oblique optical axis relative to the surface of an object, and a position detecting method using such a position detecting apparatus.
b) Description of the Related Art
A vertical detection method is known as a method of detecting the positions of marks on a wafer and a mask by using an aligner having a lens system combined with an image processing system. The vertical detection method observes position detecting marks along a direction perpendicular to the mask surface.
A chromatic bifocal method is known as a focussing method used by the vertical detection method. The chromatic bifocal method observes a wafer mark formed on a wafer and a mask mark formed on a mask by using light of different wavelengths and chromatic aberrations of the lens system, and focuses the images of the marks on the same flat plane. A wafer mark and a mask mark are hereinafter collectively called an alignment mark. An absolute precision of position detection by the chromatic bifocal method can be made high because the optical resolution of the lens system can be set high in principle.
However, since an alignment mark is observed vertically, a part of the optical system enters the exposure area. Since the optical system shields exposure light, it is necessary to retract the optical system from the exposure area when exposure light is applied. A time required for retracting the optical system lowers throughput. The alignment mark cannot be observed during the exposure, which is one of the reasons of lowering an alignment precision during the exposure.
An oblique detection method capable of solving the above-described problem associated with the vertical detection method is disclosed in Japanese Patent Laid-open Publication HEI 9-27449.
FIG. 8 is a schematic perspective view of a position detecting apparatus for the oblique detection method disclosed in the Publication HEI 9-27449. This position detecting apparatus is constituted of a wafer/mask holder unit 110 and an optical system 120.
The wafer/mask holder unit 110 is constituted of a wafer holder 115 and a mask holder 116. When position alignment is performed, a wafer 111 is held on an upper surface of the wafer holder 115 and a mask 112 is held on a lower surface of the mask holder 116. The wafer 111 and mask 112 are disposed facing each other with a predetermined gap being set between the upper surface (exposure surface) of the wafer and the lower surface (mask surface) of the mask. Wafer marks for position detection are formed on the exposure surface of the wafer 111, and a mask mark for position detection is formed on the mask surface of the mask 112.
The wafer mark 113 and mask mark 114 have edges from which incidence light is scattered. When light is incident upon these marks, light incident upon the marks are scattered whereas light incident upon another area is regularly reflected.
The optical system 120 is constituted of an image detector 121, a lens 122, a half mirror 123, and a light source 124.
The optical system 120 is disposed in such a manner that the optical axis 125 thereof is oblique relative to the exposure surface of the wafer 111. Illumination light radiated from the light source 124 is reflected by the half mirror 123 in a direction of the optical axis 125, passes through the lens 122, and becomes obliquely incident upon the exposure surface. The light source 124 is positioned at the focal point on the image side so that illumination light radiated from the light source 124 is collimated and becomes parallel light fluxes. The intensity of illumination light of the light source 124 is made adjustable.
Of light fluxes scattered at the edges of the wafer marks 113 and mask mark 114, the light fluxes incident upon the entrance pupil of the lens 122 is converged by the lens 122 and focussed on a light reception surface of the image detector 121. Since the optical axes of the illumination optical system and the observation optical system are disposed obliquely, it is not necessary to dispose each optical system just above an exposure area of the exposure surface. Therefore, exposure can be performed without retracting the exposure system from above the exposure area. It is also possible to observe alignment marks during exposure.
In the position detecting apparatus shown in FIG. 8, illumination light is applied to the alignment marks 113 and 114 after being reflected by the half mirror 123 and penetrating through the lens 122. The intensity of the illumination light decreases by about a half when the half mirror 123 reflects it. Although most of the illumination light transmits through the lens 122, a fraction of the illumination light is reflected by the surface of the lens 122. This reflected light produces flare so that the background level of light incident upon the light reception surface of the image detector 121 is raised. A contrast is therefore lowered between the background and an image formed by light scattered from the alignment marks.
Another problem is that the intensity of light propagating toward the image detector 121 is lowered by about a half by the half mirror 121, because light scattered by the alignment marks reaches the image detector 121 after passing through the half mirror 123. The intensity of light scattered and reached the image detector 121 may be raised by making the intensity of illumination light high. With this method, however, flare of illumination light increases at the same time. Therefore, this method is not effective for increasing the contrast between the background and an image formed by light scattered and reached the image detector 121.